Photovoltaic (PV) technologies have recently experienced renewed interest and corresponding technological advances, due in part to the fact that, among renewable energy technologies, PV technologies are considered to be one of the most promising as a result of their clean and environmental friendly nature and the abundance of solar energy. Conventional PV technologies typically include single crystalline silicon (c-Si) PV cells that are associated with high efficiency and stability. For example, state-of-art single crystalline silicon solar cells have power conversion efficiency of 25% and close to 20% for commercial solar panel applications. However single crystalline silicon solar cells have not replaced conventional fossil fuel energy sources due to the significantly higher cost and long energy payback time associated with single crystalline silicon solar cells. For instance, single crystalline silicon solar cells use bulk silicon wafer for active material for power generation, as well as being a supporting substrate of the entire solar cell.
However, most of the optical absorption of a typical solar cell takes place in the upper 30 micrometers (μm) surface. Accordingly, in an effort to substantially reduce the use of silicon material and associated cost of conventional silicon solar cells, thin film PV technologies have been explored. Thin film PV technologies typically use a thin layer of PV material to fabricate solar cells on a variety of substrates, which can be light weight and/or flexible, instead of using crystalline materials as the active material and supporting substrate. While research has been undertaken to improve materials and device structures of thin film PV devices in order to improve performance to a level comparable to that of crystalline solar cells, further improvements are necessary.
For instance, adding a surface texture to the front and/or the bottom surface of a PV device can be used to enhance light scattering and reduce surface reflection, in an effort to improve thin film PV device performance. As an example, nanostructures can be used to alter the optical properties of surfaces and device structures, for optical applications generally, and for PV technologies, in particular. For instance, three-dimensional (3-D) nanostructures, such as nanotubes, nanorods, nanopillars, nanocones, nanodomes, nanowires, and the like can be attractive in such applications, because 3-D nanostructures can increase the surface area relative to a planar surface of a substrate. Accordingly, the increased surface area of 3-D nanostructures, relative to the surface structure of a two-dimensional (2-D) textured substrates, can facilitate broadband light absorption and increase efficiency.
However, conventional methods of adding a surface texture and/or associated structures typically comprise fabrication methods using complicated and/or expensive processes where opportunities for cost reduction by achieving economies of scale are limited. For instance, conventional methods to fabricate 3-D nanostructures have typically focused on fabrication by vapor-liquid solid growth, photolithography, nanotransfer printing, and/or micromolding in capillaries. Thus, while the resulting 3-D nanostructures can be effective in the facilitation of broadband and efficient light trapping, such methods remain expensive and complicated, with poor controllability and scalability, thereby limiting the applicability of 3-D nanostructures in practical applications such as thin film PV technologies.
In one example, microstructures and/or nanostructures built on hard and/or rigid PV substrates such as silicon or silicon dioxide glass are not flexible, which limits their installation options and associated applications and market. Furthermore, such rigid substrate implementations typically involve batch processing, which processes are relatively expensive compared to continuous processing techniques, and which can limit the economies of scale available associated with batch processing. In another example, nanostructures (e.g., nanoconcave, nanopillar, nanocone, etc.) having 3-D structured solar cells fabricated on top of the nanostructures require special care regarding surface morphology and photon management to provide adequate performance improvements while maintaining low cost of thin film PV technologies relative conventional c-Si PV technologies. It is noted that, photon management capability of a nanostructure can depend on geometric factors, as well as material intrinsic optical properties. Accordingly, it is further noted that the ability to control geometries of nanostructures such as pitch, height, shape, etc., for example, can facilitate fabrication of optimal device structures. For instance, 3-D nanostructure such as nanopillars, nano-pyramids, etc., have been explored to improve efficiency of light trapping, but such structures are typically fabricated with the aforementioned expensive processes, where scalability is limited. As an example, structures fabricated by lithography and reactive ion etching (RIE) involve expensive equipment and associated support facilities, the maintenance of which is also expensive. Moreover, the scalability of such textured substrates, which also require batch processing, is limited by the chamber size and/or the capabilities of the process equipment, which combine to hinder the commercialization of the thin film PV technologies.
In addition, while conventional nanostructure fabrication techniques that result in non-ordered or self-ordered nanostructures can demonstrate improved photon capturing capability, such conventional nanostructure fabrication techniques can result in reductions in PV device performance for associated PV device structures. As a result, poor control of surface morphology afforded by conventional nanostructure fabrication techniques can result in reductions in PV device performance. For instance, fabrication of non-ordered (e.g., random or self-ordered) nanostructures can have irregular 3-D nanostructures (e.g., irregular morphology, pitch, spacing, ordering, etc.). As mentioned, such conventional non-ordered or self-ordered nanostructures can have improved photon capturing capability when implemented in associated PV device structures. However, irregularity on periodicity, height, etc. of non-ordered or self-ordered nanostructures can introduce large variations of coated film thickness for subsequent layers of a thin film PV device fabrication technique. Such variations can be detrimental for PV device performance, which devices can be sensitive to local variation of junction depth.
It is thus desired to provide techniques for the formation of predetermined or ordered 3-D nanostructures that improve upon these and other deficiencies. The above-described deficiencies of conventional nanostructure fabrication techniques are merely intended to provide an overview of some of the problems of conventional implementations, and are not intended to be exhaustive. Other problems with conventional implementations and techniques and corresponding benefits of the various non-limiting embodiments described herein may become further apparent upon review of the following description.